The field of the invention relates to engine air/fuel control systems.
Feedback or closed-loop control systems are known in which fuel delivery to an engine is trimmed by a feedback variable derived from an exhaust gas oxygen sensor. Under ideal conditions, feedback control maintains the air/fuel ratio within the peak efficiency window of a catalytic converter. However, converter efficiency is not maximized until the converter reaches a desired temperature.
U.S. Pat. No. 5,211,011 describes a system in which fuel delivered to the engine is alternately modulated rich and then modulated lean while a measurement of converter temperature is below a desired temperature. Ignition timing is also retarded until a converter temperature measurement reaches the desired temperature to more rapidly warm the converter. Thereafter, fuel is adjusted in response to a feedback variable derived from the exhaust gas oxygen sensor.
The inventors herein have recognized several problems with the above approaches. One problem is that air/fuel feedback control is not entered until the converter temperature reaches a desired temperature. Feedback control may thereby be delayed more than necessary resulting in unnecessary emissions. Another problem is that modulation of the fuel delivered may increase emissions. And, if fuel modulation continues during air/fuel feedback control, the range of authority of the feedback control system may be skewed thereby reducing its ability to control air/fuel ratio.